The invention generally relates to humidifying a reactant flow of a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:H2→2H++2e− at the anode of the cell, and  Equation 1O2+4H++4e−→2H2O at the cathode of the cell.  Equation 2
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.
The fuel cell stack is one out of many components of a typical fuel cell system, as the fuel cell system includes various other components and subsystems, such as a cooling subsystem, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves.
The fuel cell system may include a humidification subsystem that introduces water vapor into one or both (anode and cathode) reactant streams, or flows, that enter the fuel cell stack. More specifically, low temperature fuel cell systems, such as PEM-type systems, need reactants that are fully saturated with water vapor. The full saturation is needed to avoid drying out the fuel cells for purposes of maximizing membrane life.
Although full saturation is needed, supersaturating the reactant flow may have adverse effects. For example, a supersaturated reactant flow may flood inlets of the fuel cells. Because fuel typically is consumed for purposes of humidifying the anode reactant flow, supersaturating the anode reactant flow may also reduce the electrical efficiency of the fuel cell system.
Thus, it is typically desirable to monitor the humidification level of the reactant flow. Although a water vapor measurement probe could conceivably be used to measure the humidity level, the use of such a probe may produce insufficient results, in that the accuracy of the water vapor measurement probe in flow streams is yet unproven.
Thus, there exists a continuing need for better ways to monitor and regulate the humidification level of a reactant flow of a fuel cell system.